Castle, who last faced a primary election challenger in 1992, said the six-figure sums pumped into the state by the Tea Party Express, and the recent endorsements of O'Donnell by former Alaska Gov. Sarah Palin and South Carolina Sen. Jim DeMint, prove that his opposition is being powered by out-of-state forces.

"This has been a complete out-of-state operation, a political action committee from California, a couple people from Washington, D.C., and that's it. It's not been a local campaign. It's not had local donations," Castle told reporters in downtown Wilmington before campaigning along a bustling Market Street during the Monday lunch hour.

"It's clear they have spent several hundred thousand dollars to not only take me out but to take anybody who dares to vote with the other party at any time out," he said.

The Palin and DeMint endorsements came as no surprise to Castle, who took a swipe at the prominent conservatives.

"I was thinking about Sarah Palin and wondering if she's ever been in Delaware. I don't know if she has or not, or even been through Delaware, for all that matters," he said, noting he's never met Palin.

As for DeMint, he said, he "has made it his crusade to find the most conservative people he can support. I'm not remotely surprised that he endorsed her. I'm just surprised it didn't happen sooner or involve more money coming in."

A spokesman for DeMint said Castle's complaint about outside influences was ironic in that his campaign is being largely bankrolled by Beltway lobbyists.

"Mike Castle complains about outside money but bellies up to the bar anytime he can raise campaign cash from Washington lobbyists. Christine O'Donnell's campaign is being run on a shoestring, and the money she does have is coming from stay-at-home moms and retirees in Delaware and across the country," said Senate Conservatives Fund spokesman Matt Hoskins. "Mike Castle is about to lose a race everybody said was in the bag, and now he's looking for someone to blame other than himself."

The Tea Party Express responded by pointing to a Federal Election Commission report from the last year showing that nearly two-thirds of his contributions have come from outside the state.

"Mike Castle serves the agenda of the lobbyists of Gucci Gulch, not the interest of Delaware Republicans," said Tea Party Express spokesman Levi Russell.

Palin has cut a 60-second radio ad for O'Donnell, dubbing her a "constitutional conservative" and defending her against recent attacks on her personal finances and background.

"I can relate to the vicious personal attacks on Christine and can tell you, it's sad to see the establishment's desperation in this," Palin says over a background of patriotic music.

Castle said he believed his endorsements from former Delaware Gov. Pete DuPont and New Jersey Gov. Chris Christie, who attended the University of Delaware, would prove more valuable.

As Castle strolled down the street Monday in a dark gray suit and bright pink tie, most people greeted him casually, with "Hey, Mike" — a testament to his popularity and familiarity in this tiny state.

One man driving a pickup truck yelled, "OK, Mikey!" as he waved back. Another woman greeted him with "I saw you on TV last night."

Standing on the street corner, Castle took a passive approach to his campaigning. Rather than advance toward voters, the soft-spoken veteran congressman and former governor politely accepted their greetings as they walked up to him in front of a collection of reporters and television cameras.

"I know, you always have," said Leonard Simon, a Castle supporter and lifelong state resident.

When asked later what he thought of O'Donnell, Simon replied, "Basically delusional."

"You've got a delusional candidate that's supported by somebody who lives in Alaska. This is still Delaware," he said. "Especially at this end of the state. I think she'll have some support down in sort of more of the redneck area downstate."

A survey released Sunday evening by Public Policy Polling showing O'Donnell holding a 3-point lead over Castle on election eve spooked Republicans in Delaware and Washington, but Castle said he was comfortable with his campaign's internal numbers.

Despite his calm demeanor, he hinted that he was a bit outside of his comfort zone by engaging O'Donnell through negative ads on radio and television.

"It's not what I would prefer to do in a campaign. On the other hand, I've never had an opponent who has the deficiencies of this particular opponent, and I think that needed to be pointed out, in terms of no job and paying for her own living expenses from her campaign and a whole variety of other things," Castle said.

"It's about a very extreme candidate running a very extreme race, with the idea that if you don't vote ideologically with everything she and her followers believe in, then you need to go. It's about a little more than just me," he continued.

Castle highlighted a message about electability, warning that nominating O'Donnell would throw the election to presumptive Democratic nominee Chris Coons.

"I've yet to see a poll that shows her having any chance of winning the general election. The bottom line is, if she's nominated, Republicans lose the election automatically. It's that simple," he said.

Castle wouldn't directly address a question about whether he would support O'Donnell if he came up short Tuesday but signaled it would be very difficult.

"That's very hypothetical, and I will answer it at some point in the future. If it were to happen, I would look at that. As you know, this has not been a very pleasant experience for anybody, and the concept of supporting one another in this circumstance is going to be difficult for anyone to reach," he said.

The O'Donnell campaign did not respond to numerous requests for comment.